1. Field of the Invention
The present invention relates to an electrophotographic apparatus such as a printer, copier or facsimile for forming an image according to electrophotographic system.
2. Related Art
Image forming apparatuses which make use of electrophotographic system, such as laser printers and LED printers are now widely used. Along with the spread of personal computers, low-cost printers are now in demand. Laser beam system is characterized by easier exposure control in comparison with LED system and can provide an inexpensive image forming apparatus.
The processing of a photographic image by an individual user is made possible by a reduction in the cost of a high-performance personal computer, and a high-resolution printer capable of outputting the processed photographic image is desired. Meanwhile, as progress is being made in the networking of offices to enable a plurality of users to use a printer at the same time, high-speed printers are also in demand.
Thus, when the resolution and the process speed are high, the revolution speed of a scanner increases in a laser beam printer, whereby a large driving power source is required and a rotary support shaft must be reinforced, thereby increasing the size of the scanner or apparatus. Since the emission time per pixel becomes short, a high-output laser chip is required, thereby boosting the cost of the apparatus.
To solve the above problems, multi-beam system is effective that a plurality of laser beams are used for scanning to form an electrostatic latent image of a plurality of lines on an electrophotographic sensitive body.
Most of inexpensive charging devices for charging an electrophotographic sensitive body employ contact charging system. This system does not require a high-voltage power source and an ozone filter, and a charging member of this system may be simply composed of a roller. As this contact charging system, the applicant of the present application has proposed system that voltage to be applied to the contact charging member is oscillating voltage (Japanese Patent Publication No. 3-52058). In this system, the peak value of oscillating voltage is set to two times or more the charging start voltage when DC is applied to the charging member to enable uniform charging.
According to studies conducted by the present inventor, when a contact charging member for applying oscillating voltage is used to charge an electrophotographic sensitive body and the resolution and the process speed are increased, a defective image called xe2x80x9cmoire imagexe2x80x9d is obtained upon the output of a line image. This is an interference fringe which is formed when periodical slight potential fluctuations caused by the oscillating voltage of charging are close to the pitch of the line image. This moire image will be described hereinbelow.
When the process speed is represented by V (mm/sec), the frequency of charging voltage (AC component) is represented by f (Hz), the density (resolution) of pixels is represented by D (dot/mm), the number of dots of the line width of the line image is represented by N, the number of dots between lines of the line image is represented by M, the line pitch is represented by L (=(N+M)/D) (mm/cycle) and the value of space frequency calculated from the charging frequency f is represented by fp (=f/V) (cycle/mm), if the space frequency fp (cycle/mm) of charging and the space frequency 1/L (cycle/mm) of an image are close to each other, an interference fringe which is formed in an image of arbitrary N and M having the relationship of the following equation (1) is called xe2x80x9cmoire imagexe2x80x9d.
fp=f/V=1/L=D/(N+M)xe2x80x83xe2x80x83(1)
To prevent this phenomenon, the space frequency of charging frequency is made much larger than the space frequency of an image or set to a frequency other than the space frequency of the image in the prior art.
However, as is understood from the equation (1), when the resolution D is high, the charging frequency f which can avoid the moire image becomes high. For example, when a moire image can be avoided by setting the charging frequency to 600 Hz at 400 dpi, a charging frequency of 1,800 Hz is required at 1,200 dpi. However, when the charging frequency is set to a high level, charging sound becomes big corresponding to frequency disadvantageously.
It is also understood from the equation (1) that when the process speed (mm/sec) is high, the charging frequency must be made high.
It is an object of the present invention to provide an electrophotographic apparatus which prevents a moire image.
It is another object of the present invention to provide an electrophotographic apparatus which can reduce charging sound.
It is still another object of the present invention to provide an electrophotographic apparatus which enables uniform charging even with the increased resolution and process speed.
It is a further object of the present invention to provide an electrophotographic apparatus in which the repeatability of small-dot image formed of 1 dot or several dots is improved.
It is a still further object of the present invention to provide an electrophotographic apparatus which can form an electrostatic image having a big difference of potential even with the large spot diameter of an exposure beam.
Other objects and features of the present invention will become more apparent from the following description taken in connection with the accompanying drawings.